1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
U.S. Pat. Nos. 5,339,737 and 5,379,698 disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge (see, e.g., U.S. Pat. No. 5,385,092 and U.S. application Ser. No. 08/376,766, the entire disclosures of which are hereby incorporated by reference). These include "wet" plates that utilize fountain solution during printing, and "dry" plates to which ink is applied directly.
All of the disclosed plate constructions incorporate materials that enhance the ablative efficiency of the laser beam. This avoids a shortcoming characteristic of some prior systems, which employ plate substances that do not heat rapidly or absorb significant amounts of radiation and, consequently, do not ablate (i.e., decompose into gases and volatile fragments) unless they are irradiated for relatively long intervals and/or receive high-power pulses. The disclosed plate materials are all solid and durable, enabling them to withstand the rigors of commercial printing and exhibit adequate useful lifespans.
In one disclosed embodiment, the plate construction includes a first layer, an imaging layer that ablates when exposed to a pulse of imaging (preferably infrared, or "IR") radiation, and a substrate underlying the imaging layer. The first, topmost layer is chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid, while the substrate is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Exposure of the plate to a laser pulse ablates the imaging layer, weakening the topmost layer as well. As a result of ablation of the second layer, the weakened surface layer is no longer anchored to an underlying layer, and is easily removed in a post-imaging cleaning step. This creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
As disclosed in the '698 patent, a thin layer of metal, preferably titanium, can be used as an ablation medium. Destruction of the titanium layer, which intervenes between an overlying top layer and a substrate, leaves the top layer unanchored and therefore vulnerable to removal by cleaning. The '698 and '737 patents, whose entire disclosures are hereby incorporated by reference, also disclose lamination of the substrate to a sturdy metal support.
Metal imaging layers are well-suited to environments (such as plate-winding arrangements) that require substantial flexibility, since the metal can be applied at miniscule thicknesses. Titanium is preferred as a metal ablation medium because it offers a variety of advantages over other IR-absorptive metals. Titanium layers exhibit substantial resistance to handling damage, particularly when compared with metals such as aluminum, bismuth, chromium and zinc; this feature is important both to production, where damage to the imaging layer can occur prior to coating thereover of the top layer, and in the printing process itself where weak intermediate layers can reduce plate life. In the case of dry lithography, titanium further enhances plate life through resistance to interaction with ink-borne solvents that, over time, migrate through the top layer; other materials, such as organic layers, may exhibit permeability to such solvents and allow plate degradation. Moreover, silicone coatings applied to titanium layers tend to cure at faster rates and at lower temperatures (thereby avoiding thermal damage to the underlying substrate), require lower catalyst levels (thereby improving pot life) and, in the case of addition-cure silicones, exhibit "post-cure" cross-linking (in marked contrast, for example, to nickel, which can actually inhibit the initial cure). The latter property further enhances plate life, since more fully cured silicones exhibit superior durability, and also provides further resistance against ink-borne solvent migration. Post-cure cross-linking is also useful where the desire for high-speed coating (or the need to run at reduced temperatures to avoid thermal damage to the plate substrate) make full cure on the coating apparatus impracticable. Titanium also provides advantageous environmental and safety characteristics: its ablation does not produce measurable emission of gaseous byproducts, and environmental exposure presents minimal health concerns. Finally, titanium, like many other metals, exhibits some tendency to interact with oxygen during the deposition process (vacuum evaporation, electron-beam evaporation or sputtering); however, the lower oxides of titanium most likely to be formed in this manner (particularly TiO) are strong absorbers of near-IR imaging radiation. In contrast, the likely oxides of aluminum, zinc and bismuth are poor absorbers of such radiation.
Despite their advantage in many printing environments, metal imaging layers do exhibit one shortcoming relative to IR-absorptive polymeric layers: the latter can be loaded with radiation-absorptive materials (e.g., carbon-black pigment) that render the layer capable of absorbing nearly all incident energy from an imaging pulse. A titanium metal layer, by contrast, will absorb a smaller fraction of an imaging pulse, transmitting and reflecting at least some pulse energy. As a very rough example, my work suggests that a titanium layer produced in accordance with the '698 patent absorbs approximately 50% of an imaging pulse, transmitting about 30% and reflecting about 20%. By contrast, it is possible to design nitrocellulose imaging layers loaded with carbon black that absorb 90% or more of an imaging pulse.
The result of this limited absorption is the need for relatively high pulse energies. The laser-driven imaging apparatus noted above operates by focusing the laser beam to a desired spot size on the printing member. The power of the laser is chosen such that this spot possesses an energy density adequate to achieve ablation. Deviation from proper optical alignment (resulting from vertical movement above and below the focused distance) produces a broader spot, i.e., a less concentrated beam having a correspondingly smaller energy density. Depth-of-focus in this type of imaging apparatus refers to the tolerable deviation from the chosen spot size--that is, the maximum degree of beam spread that will still achieve ablation. Thus, delivered pulse energies can be increased to accommodate limited-absorption imaging layers by utilizing higher-powered lasers or by designing an optical system that will maintain a precise focus and thereby reduce the necessary depth-of-focus tolerance.
Neither of these options is desirable, however, since higher power requirements can substantially increase laser cost, while reducing depth-of-focus tolerance places substantial demands on mechanical design; the required precision is particularly difficult to maintain in rigorous commercial platemaking and, especially, printing environments.
A better approach is to increase the fraction of energy absorbed by the thin-metal imaging layer. In one type of construction, described in the '698 patent and also in U.S. Pat. No. 5,570,636 entitled LASER-IMAGEABLE LITHOGRAPHIC PRINTING MEMBERS WITH DIMENSIONALLY STABLE BASE SUPPORTS, radiation is reflected back into the thin-metal imaging layer by an underlying reflective metal layer. In this way, the energy transmitted through the imaging layer is reflected back into that layer, substantially increasing the net energy available for absorption.
Ordinarily, this type of construction requires an intervening layer between the imaging and reflective layers, since these tend to be thermally conductive. Direct application of a titanium imaging layer, for example, to an aluminum support will in most cases prevent formation of an image due to conduction of heat through the support, which prevents sufficient energy from building up in the titanium layer to cause its ablation. Such conduction loss is avoided in the laminated constructions contemplated in the '698 and '737 patents due to the presence of an intervening polymeric substrate and layer of laminating adhesive, and in the '636 patent (the entire disclosure of which is hereby incorporated by reference) by introduction of a thermally insulating layer between the reflective layer and the thin-metal imaging layer.
The need for a separate reflective layer not only adds material cost and manufacturing overhead to the final plate, but can reduce its flexibility. A flexible plate is essential for plate-winding arrangements such as those disclosed in U.S. Pat. No. 5,355,795 and U.S. application Ser. No. 08/435,094 (filed on May 4, 1995 and entitled REMOVABLE SUPPLY AND UPTAKE ASSEMBLIES FOR LITHOGRAPHIC PLATE MATERIAL). Indeed, the use of a heavy aluminum support to provide reflection capability, as described in the '636 patent, is fundamentally incompatible with such arrangements.